Photopolymerizable and/or photocurable and/or radiation curable resins can be selectively cross-linked and cured to create relief images during the production of relief image printing plates (including flexographic printing plates). These images are also desirable in other applications, including the production of signboards, such as plaques, display panels, decorative shields, same plates, Braille boards and the like and the combination of one or more of the foregoing. These resins can also be used making master relief patterns for the production of molds, to produce castings, including bronze and other metal castings.
A common approach currently used to make flexographic printing plates utilizes a photopolymerizable, also known as a photosensitive, photoimageable, photopolymer, photohardenable or photocurable resin composition. While many different photopolymerizable resin compositions are known, they share the quality that upon exposure to certain wavelengths of light, the resin reacts with itself to form a structure that is insoluble in ink. Thus, photopolymerizable resin compositions may be used to form a hard, ink-insoluble, raised surface (i.e., relief image) for photopolymer plates.
Photopolymerizable resins can be processed either as a solid plate made of a cast photopolymerizable resin or by forming a layer of a liquid photopolymerizable resin and exposing the formed resin layer to actinic radiation followed by developing to remove uncured portions of the photopolymerizable resin layer. Liquid photopolymerizable resins provide an advantage over solid sheet polymers because the uncured resin can be reclaimed from the non-image areas for later reuse. Liquid photopolymerizable resins have a further advantage as compared to sheet polymers in terms of flexibility to enable the production of images of widely varying thicknesses simply by changing the machine settings.
In a typical process for preparing a printing plate or other relief image, an image-bearing transparency or negative, i.e., a transparent film having opaque regions corresponding to the reverse of the image which one desires to impart to a printing plate, is placed on a glass platen, and covered with a transparent, polymeric coverfilm and a liquid photopolymerizable resin is released from a resin reservoir and is cast onto the cover film with the aid of a leveling blade. The image-bearing transparency and coverfilm are secured by vacuum to the platen so that air is displaced and bubbles are not created in the liquid photopolymerizable layer. Substantially simultaneously with casting the liquid photopolymerizable resin, a substrate, such as a polymeric film, is placed on the layer of photopolymerizable resin. A second source of actinic radiation may be positioned above the substrate. In this instance, a glass platen is positioned between the substrate and the second source of actinic radiation to provide a means for controlling thickness uniformity of the layer of photopolymerizable resin by lowering the glass platen, prior to exposure, to rest on shims and make contact with the substrate.
The liquid photopolymerizable resin is then exposed to actinic radiation to crosslink and cure the photopolymerizable resin and produce the relief image printing plate or other relief image product. The exposure can be a blanket exposure, in which the entire surface is exposed to actinic radiation (i.e., to create the floor layer) or may be an imagewise exposure in which the photopolymerizable floor layer is exposed through a film negative or other photomask.
The photopolymerizable resin is imaged by exposing the cast photopolymerizable resin to actinic radiation through the film negative or other photomask to selectively crosslink and cure the photopolymerizable resin. The image is typically exposed from at least one side of the plate, preferably the front side, and in some instances may be imaged from both the top and bottom as discussed above. Actinic light, such as UV light, is directed against the photopolymerizable resin layer through the film negative or other photomask. The result is that the liquid photopolymerizable resin is selectively cross-linked and cured to form a printing image surface that mirrors the image on the negative. Upon exposure to actinic radiation, the liquid photopolymerizable resin polymerizes and changes from a liquid state to a solid state to form the raised relief image.
The regions of the resin layer which were protected from the actinic radiation by the opaque regions of the transparency are washed away using a developer solution. The cured regions are insoluble in the developer solution, and so after development a relief image formed of cured photopolymerizable resin is obtained. The cured resin is likewise insoluble in certain inks, and thus may be used in flexographic printing as described above. U.S. Pat. No. 2,760,863 to Plambeck describes another typical process for preparing a printing plate using photopolymerizable resin, wherein the image-bearing transparency is placed above the photopolymerizable layer, rather than underneath the coverfilm.
A common variation on the above-described process is to expose a liquid photopolymerizable resin to actinic radiation from two sides of the resin layer. See, for example, U.S. Pat. No. 3,848,998 to Yonekura et al. The recognized advantages of exposing from the back (through a backing sheet) as well as the front (through an image-bearing transparency) include better adhesion of the photopolymerizable composition to the backing sheet, better relief image formation, and overcoming the inhibition to polymerization of photopolymerizable resin that is exposed to oxygen with increased control over the relief image height.
In all areas not exposed to UV radiation, the photopolymerizable resin remains liquid after exposure and can then be recovered and reclaimed. In a typical process, the uncured photopolymerizable resin is physically removed from the plate in a further process step such that it can be reused to make further plates. Any residual traces of liquid photopolymerizable resin remaining are then removed by nozzle washing or brush washing using a wash-out solution to obtain a washed-out plate, leaving behind the cured relief image. This reclamation technique not only saves material costs of the photopolymerizable resin but also reduces the use and cost of developing chemistry and makes a lighter relief image plate that is safer and easier to handle.
Residual traces of liquid photopolymerizable resin remaining in the regions of the photopolymerizable resin which were protected from the actinic radiation by the opaque regions of the image or film negative can then be washed away using a developer solution or by other development means as is known in the art. The cured regions are insoluble in the developer solution. Thereafter, the cured printing plate may be subjected to various post exposure steps. For example, the plate may be completely immersed in water and exposed to actinic radiation such as UV light emitted from a light source to perform a complete curing of the entire plate and to increase plate strength. After development, a relief image formed of cured photopolymerizable resin is obtained.
Liquid platemaking processes are also described, for example in U.S. Pat. No. 5,213,949 to Kojima et al., U.S. Pat. No. 5,813,342 to Strong et al. and U.S. Patent Publication No. 2008/0107908 to Long et al., the subject matter of each of which is herein incorporated by reference in its entirety.
U.S. Patent Publication No. 2003/0111771 to Gybin et al., the subject matter of which is herein incorporated by reference in its entirety, describes a mold making process for making cast objects. Gybin describes a method of forming a mold that can be used for making customized molded objects or making low-volume molded components. A photosensitive film laminate, including at least one layer of photosensitive material, is selectively exposed to actinic radiation and a portion that corresponds to either the exposed or unexposed portion of the photosensitive material is removed. Removal of this portion forms a relief surface in the laminate film and a casting material can then be applied to relief surface to form a cast article. Suitable casting materials include, but are not limited to plaster, resins, and low melting temperature metal alloys.
U.S. Pat. No. 4,668,607 to Wojcik, the subject matter of which is herein incorporated by reference in its entirety, describes another process for using photosensitive resins in the production of casting molds, particularly for use with cast metal, including bronze which includes forming a master mold and then a female mold from the master mold. In the fabrication of bronze castings, including for example, decorative or commemorative plaques and the like, a female mold is usually prepared into which bronze or other metal is poured.
As described in Wojcik, generally the first exposure is a backside imagewise exposure followed by an overall exposure through the backside (i.e., “backflash”) and frontal imagewise exposure through a film negative or other photomask which bears the image or design desired. After exposure, the unhardened areas of the layer are removed by solvent development to form the master. The master is then used in mold casting whereby a mixture of mold-making material, e.g., comprising sand and wax as known to those skilled in the art, is packed over the master to replicate a complementary relief pattern, and, following removal of the master, a female mold results which is suitable for casting of a metal decorative plaque or other article. In order to produce such a decorative metal article, molten bronze or other known casting metals, e.g., silver, aluminum, gold, platinum, copper, etc. is cast into a mold containing a complementary recessed pattern of the raised pattern found in the plaque.
The typical method of imagewise exposing and developing liquid photopolymerizable resins produces relief images in which the relief comprises a raised pattern of cured photopolymer, and the raised pattern comprises essentially flat top surfaces of the raised pattern and sharp edges at the point where the top surface of the raised pattern meets the sidewall of the raised pattern. This is typical of curing against a flat surface or under impression by a flat surface as is done is liquid photocuring and produces a desirable result, especially for relief image printing in which a substantially planar printing surface is necessary to produce a good result.
In other applications however, rounded/smooth image edges would be preferable and desirable, such that there is not as sharp edge at the point where the top surface of the raised pattern meets the sidewall of the raised pattern. Rounded/smooth image edges are often desired, for example, in signage applications, bronze casting and molding applications, and applications which involve the formation of Braille dots.
Thus, it would be desirable to provide a method of curing liquid photopolymerizable resins to produce a relief image having rounded and/or smooth edges.